Neutron spectrometry can involve the generation of a polychromatic beam of neutrons which are characterized by different velocities and hence energies, these velocities and energies being associated with respective frequencies.
Gnerally a monoenergetic beam of neutrons is desired and it is thus a common practice to subject the polychromatic beams to a form of filtration or monochromatization, selecting the neutrons of a single energy, velocity or frequency from the polychromatic beam.
The monochromatic beam can be used for the investigation of solid matter by neutron scattering or for activation of a substance or for investigations of the dynamic behavior of atoms. In all of these cases a so-called monochromatic or monoenergetic beam of neutrons is more desirable than the "white" or polychromatic beams produced by simple collimation of the output of a neutron generator, e.g. a nuclear reactor or pile.
The coarse monochromatization or energy-separation of the polychromatic beam of neutrons is generally carried out by drum separators. Such separators provide a rough separation of the beams so that the beam passing the separator is monoenergetic within certain limits. Such devices are also known as energy or velocity selectors. In a typical energy or velocity selector of the drum type, the drum consists of neutron absorbing material and is provided with axial and helical grooves such that only neutrons within a specific velocity range can pass, all others being absorbed on the drum.
To eliminate harmonics, disk choppers are provided, the choppers having disks of neutron-absorbing material and provided with windows through which the neutrons can pass. The drums and disks can be arranged in succession and, of course, the more disks, drums and the like provided along the neutron path, the greater will be the resolution and hence the energy band width of the beam reaching its ultimate destination or target. This increased "definition" is found to be in the form of lines of distinct wavelength and hence a limited spread of the energy spectrum of the beam reaching the target etc.
In the present application, the term "frequency" when referring to a neutron beam will be understood to be the reciprocal of the wavelength .lambda. where .lambda. = h/mv = (h/p). In this relationship h is Planck's constant, p is the momentum of the neutron, m is the neutron mass and v the neutron velocity. The neutron energy can be determined by the Einstein relationship from the mass and velocity.
In such choppers and other components of time-of-flight neutron spectrometers, it is desirable to provide a system for determining the angular position of the rotor of a synchronous motor relative to the phase position of the rotary field driving the synchronous motor.
Such devices or systems are advantageous, for example, for the measurement of the load angle of a synchronous motor either purely for the purpose of measurement (e.g. to permit plotting of the load-angle characteristic) or for using the measured value of the load angle as the input parameter (instantaneous value) for a control circuit regulating the operation of the motor.
In time-of-flight neutron spectrometers it is important to establish the rotation angle of the rotor relative to the input frequency of the frequency generator energizing the motor (i.e., the degree to which the rotor may be lagging the position determined by the applied frequency) or to measure relative angular positions of the rotors of several synchronous motors.
It is known to determine the angular position of the pole rotor of a synchronous motor by means of a stroboscope triggered at the frequency of the frequency generator or transmitter. It is disadvantageous with such systems that the measurement can only be read optically and is not represented by an electronic signal that can be used as an input for control or other purposes.
It has also been proposed to provide a pulse-counting circuit with automatic start-stop means and associated with an optoelectronic device. The pulse counter is triggered by a pulse from the frequency generator or transmitter and it is stopped by a further pulse from the optoelectronic device when a zero mark carried by the magnetic-pole rotor is aligned therewith.
The pulse count obtained within the time interval between the start and the stop pulse is stored in the counter. The count pulses are produced by the optoelectronic device from subdivision traces or marks carried by the rotor. The disadvantage of this arrangement is that precision of the measurement is limited by the low resolution capacity of an optoelectronic device when the rotor is driven at high speed.